Solid state lighting (SSL) is rapidly becoming the norm in many lighting applications. This is because SSL elements such as light emitting diodes (LEDs) can exhibit superior lifetime and energy consumption, as well as enabling controllable light output color, intensity, beam spread and/or lighting direction.
Tubular lighting devices are widely used in commercial lighting applications, such as for office lighting, for retail environments, in corridors, in hotels, etc. A conventional tubular light fitting has a socket connector at each end for making mechanical and electrical connection to connection pins at each end of a tubular light. Conventional tubular lights are in the form of fluorescent light tubes. There is a huge installed base of luminaires equipped with electronic ballasts for fluorescent light tubes. The ballast circuit is external of the light tube, and comprises a ballast (inductor) and a starter circuit. The ballast, starter circuit and the two pairs of connection pins from a closed circuit. In a conventional fluorescent light tube, a heating filament between the connection pins of each pair completes the circuit.
There are now tubular LED (“TLED”) solid state lamps which can be used as a direct replacement for traditional fluorescent light tubes. In this way, the advantages of solid state lighting can be obtained without the expense of changing existing light fittings.
FIG. 1 shows a basic known tubular solid state lamp 10, comprising a tubular housing 12 having an end cap 14 at each end (only one is shown). The end cap 14 carries external connectors 16 in the form of two pins offset to each side from a central axis of the end cap 14, parallel to an elongate axis 15 of the tubular housing 12. The end cap 14 connects electrically to the internal driver board and the circuit board which mounts the solid state lighting elements, for example LEDs, inside the tubular housing 12.
FIG. 2A shows the basic circuit of a standard fluorescent light tube luminaire. It comprises a glow starter 17, ballast 18 and the mains AC source 19. Together with filament wires bridging the pairs of contact pins at each end of the tube 10, a closed circuit is formed. A basic electromagnetic (EM) ballast such as shown in FIG. 2A may operate at mains frequency, whereas an electronic ballast has electronic components to operate at a high frequency, such as 20 kHz.
FIG. 2A illustrates how it is safe to touch the non-connected end of the tube for a fluorescent light tube. A conventional fluorescent light tube can be inserted into such a live mains fixture without any danger because the connection pins on either side of the lamp are electrically insulated from each other by the glass tube of the lamp and the gas inside it. An electrical contact between the two ends of the lamp is only established if the gas inside it is ignited and this is only possible after both ends of the lamp have been inserted into the luminaire.
Taking the lamp out of the luminaire will immediately stop both the current flowing through it and the gas discharge in it and thus immediately re-establish electrical insulation between both ends of the lamp.
However, inserting a TLED lamp into a luminaire is potentially dangerous since it is possible to touch the connection pins on one end of the lamp whilst the other end of the lamp is already inserted and in contact with a hazardous voltage.
A typical TLED retrofit lamp contains LED PCBs and LED driver PCBs which offer little electrical insulation between the connection pins on both ends of the TLED. It may therefore be dangerous to insert such a TLED into a live mains fixture because there is a conductive path between the two ends of the tube.
Various pin safety measures have been proposed to overcome this safety issue. These pin safety measures usually interrupt the electrical connection between both ends of the TLED by at least one switch that is only closed when both ends of the TLED are inserted into the luminaire.
Both electrical and mechanical pin safety mechanisms are known.
In one known electrical pin safety solution, power is only taken from a first side of the tube and the other side is isolated from the first, and is arranged as a short between the two pin connections on that other side. The glow starter 17 (FIG. 2A) has to be replaced by a dummy starter with a bridging wire or a fuse inside, so that the loop for the current is closed.
This method has its limitations since it only works with lighting fixtures which contain a starter, these fixtures may be known as an electromagnetic (EM) fixture as they commonly utilize an EM ballast. (FIG. 2A). For the rapid starter fixtures (FIG. 2B and FIG. 2C) there are no starters in the circuit and therefore the dummy starter method does not work. For rapid starter fixtures, and for some other types of ballast, other pin safety solutions are required. Different territories have different preferred types of ballast although the use of other types of ballast in the territories is possible. For example, the North American region favors the T12 lamp with a rapid start ballast.
For example, in some other electrical pin safety solutions, an electromagnetic relay is closed when both ends of the TLED are inserted into the lamp holders in the luminaire. Insertion of the TLED into the luminaire is detected and the electromagnetic relay is closed using currents and voltages originating from the electronic ballast. An advantage of the relay pin safety solution is that it is fool-proof and maintains the look and feel of a normal lamp.
Issues related to electrical pin safety mechanisms are the compatibility with the large number of different types of electronic ballasts and the cost and reliability of electromagnetic relays.
In mechanical pin safety solutions, at each end of the TLED a switch is closed when pressing a button. Either the lamp holder will push the button when inserting the TLED into the luminaire or it needs to be pressed manually. This can be used for all types of ballasts, but it changes the way the TLED has to be installed, and it may not be compatible with all different luminaire and socket mechanical designs, since this depends on the button design and the luminaire and socket design.
As mentioned above, some electrical safety solutions may be suitable only for certain types of ballast. For example, providing the driver at one end only of the lamp may not be possible for high frequency (HF) electronic ballast (including rapid start electronic ballast) and for rapid start low frequency (EM) ballast. In particular, there is no way to place a dummy starter to close the circuit. A short at one end will also short out a heating voltage across the pins.
FIG. 2B shows a rapid start single lamp, and FIG. 2C shows a rapid start dual lamp. The lamps are for example T12 lamps (standard 38 mm diameter tubes).
The ballast in the rapid start configuration is based on an autotransformer 20, which converts the mains voltage to a suitable voltage for driving the lamp. Furthermore, its internal impedance regulates the lamp current. The ballast has heating windings, which provide current to heat the filament of the fluorescent light tube, thus reducing the lamp starting voltage and extending the lamp lifetime. The ballast usually also has an internal power factor compensation capacitor 22. There is no starter 17 needed in a rapid start fixture, which means the pin safety solution explained above in which a glow starter is replaced with a dummy starter cannot be applied and a new pin safety solution is required for the rapid start EM ballast.
U.S. Pat. No. 8,917,020 discloses a relay pin safety solution designed for rapid start EM ballasts (as commonly used in North America).
FIG. 3 shows the general configuration. The LED board 30 is connected to driver PCBs 32,34 at each end. The driver PCB 32 has a relay 32a and a driver circuit 32b and the driver PCB 34 has a relay 34a and a driver circuit 34b. 
The relay contacts provide pin safety. The filament heating voltage (from the ballast) is converted to a dc voltage by a rectifier 32c, 34c and actuates the relay contact. The relay stays open until the lamp is fully inserted in the fixture. Therefore it is safe to touch the pins when the opposite side is inserted in the lamp holder and energized. There are two relays used in the lamps, one for each end.
This solution works well, but the drawback is that two relays are needed which gives rise to additional cost. A further complication is that there are certain variations on the filament heating voltage depending on ballast type and mains voltage variation. A most simple choice of rectifier is thus not able to guarantee a stable voltage to ensure that the relay contact is firmly closed.
An alternative design makes use of a relay and an optocoupler as disclosed in WO2013/150417. FIG. 4 shows the configuration.
Each end is coupled to a rectifier 40, 42. A single relay 44 is needed and an optocoupler 46 connects the two ends. The heating voltage at the left end is converted to a suitable voltage for driving the relay coil 44 for example by a boost converter 47. The heating voltage from the right end energizes the primary side of the optocoupler and the output of the optocoupler enables the relay coil voltage, so that the relay contacts closes. With such a circuit, the main current can only flow (under the control of the driver circuit 48) when both sides of the lamp are fully inserted in the luminaire. If only one side of the lamp is inserted, the relay coil will not receive a voltage and the open contact provides pin safety.
The additional power converter 47 stabilizes the relay coil voltage and thus provides reliable operation of the relay. This arrangement still needs a relay and associated control circuitry.
The existing solutions are thus expensive and large in size because of the relay component. Furthermore the relay has a limited switching lifetime because of the moving parts and contact surface damage due to arcing. The circuit required for controlling the relay is also complex.
There is therefore a need for an improved system for providing protection when fitting a TLED to a luminaire which can be implemented with low cost and reliable circuitry, and in particular which is functional for existing ballasts with electrode heating (i.e. rapid start) circuitry.